The broad, long-term objective of this research is to understand the mechanisms regulating the differentiation and function of type II alveolar epithelial cells in lung. The immediate objectives are to determine the structure and function of a protein (p146) expressed on the plasma membrane of selected epithelial cells, and to determine the factors which regulate its expression during lung development. The p146 protein was unrecognized until identified by a monoclonal antibody, and its physiological function is unknown. Preliminary data suggest that the p146 protein represents a marker of type II epithelial cell differentiation which is independent of the surfactant system. Such a marker is important because it would simplify the distinction between factors which promote differentiation of the type II epithelial cells and factors which regulate surfactant synthesis in these cells. In lung, the p146 protein is expressed only on the apical surface of the type II alveolar epithelial cells. The molecule is also present on the microvillous surface of the renal proximal tubular epithelial cells and the luminal surface of the small bowel epithelium. This polarized distribution of the protein in specific types of epithelial cells suggests that the presence of the protein might be associated with common physiological function of these tissues. The specific aims of the research are: (1) Develop immunoassays for use with a fetal lung explant system to quantify the amount of p146 antigen present in lung at different developmental stages and to delineate the factors which are important in regulating its induction in the fetal lung. (2) Develop complementary DNA probes corresponding to the p146 protein and use the probes to characterize and assay the mRNA coding for the protein at different developmental stages and under different experimental conditions. (3) Compare the regulation of p146 protein to the regulation of surfactant apoprotein (SAP-35), a currently recognized marker of type II epithelial cell differentiation. (4) Deduce the primary sequence of the p146 protein by cloning and sequencing of complementary DNA corresponding to the complete amino acid sequence. (5) Compare the amino acid sequence of the p146 protein with sequence information available in current databases to determine if similarity or homology with known proteins can be established.